Fix the remaining cases of <prog>_CFLAGS -> <prog>_CPPFLAGS [v9.18]
Building BIND 9 with older version of BIND 9 installed would result in build failure. Fix the last two remaining cases where _CFLAGS was being used leading to wrong order of the build flags on the command line.
(cherry picked from commit 41a60a0e)
Closes #3203 (closed)
Edited by Ondřej Surý